INTERNATIONAL SATELLITE DIRECTORY
Navstar
The US Navstar Global Positioning
System (GPS) will provide continuous,
worldwide, all-weather, three-dimensional
position, velocity, and time. Full three-
dimensional service (latitude, longitude,
and altitude) will become available in
1989 when the planned constellation of 18
Navstar satellites (plus three in-orbit
spares) is complete. Two-dimensional
service (latitude and longitude) will be
available from 1987, once 12 satellites are
in orbit. Rockwell receiveda$l,170 million
multi-year contract in 1983 to build 28
Block II (operational) satellites, seven to
be produced and launched annually until
21 are in orbit in six orbital planes, with
three active Navstars in each plane. Block
II Navstars have a secondary mission to
detect and locate atmospheric nuclear
explosions, superseding the purpose-built
Vela satellites. The military precise posi
tioning service, protected by encryption,
will provide a location accuracy of within
15m SEP (spherical error possible),
18- lm horizontally and 29-7m vertically,
velocity to within 0 • lm/sec, and time to
within 100 nanosec. The commercial stan
dard positioning service will provide
location to within 60m SEP, 100m hori
zontally and 162m vertically, but can be
changed.
Purpose Navigation satellite
Operation Each Navstar satellite
continuously transmits navigation signals
on two frequencies: 1,575-42MHz and
1,227-6MHz. Superimposed on these
frequencies are two coded signals unique
to each satellite: a precision (P) signal and
a clear/acquisition (C/A) signal of twice
the power. Each signal contains the
satellite's identity, its position in space,
and the time at which the signal was
transmitted. The GPS receiver notes the
time at which the signal is received and
from this calculates range to the satellite.
Measurements from four satellites are
required to determine the four unknowns
of user latitude, longitude, altitude, and
clock bias (the time-keeping error
between satellite and user). User velocity
is determined from the Doppler shift of
the received signals. The 18-satellite
constellation ensures that four Navstars
will be visible almost all of the time.
Owner US Department of Defence
Manufacturer Rockwell International
Orbit Circular, 20,165km high, six orbit
planes, 55° inclination
Launch/status
Block 1 (first 11 satellites)
Navstar I Feb 1978
II May 1978
III Oct 1978
IV Dec 1978
V Feb 1980
VI Apr 1980
VII Dec 1981/lost on launch
VIII May 1983
IX Jun 1984
X Sep 1984
XI
Block 2 (28 satellites) seven a year
starting in 1986
FLIGHT International, 12 January 1985
Above Rockwell International has a $1,170 million contract to build 28 Block 2 operational Navstar
global positioning system satellites. Below Hughes Communications Services is to operate four
Leasat communications satellites, leased to the US Navy as Syncom IVs
Stabilisation 3-axis
Power (end of life) Block 1 0-4kW
Weight (at launch) Block 1 770kg
Block 2 1,715kg
Design life Block 1 5 years
Block 2 7-5 years
Cost $l,170m (1983)-28 Block 2
Remarks Block 2 Navstars have four
highly accurate atomic clocks—two caesium
and two rubidium. Operational Navstars
will also carry the integrated operational
nuclear detection system (Ionds).
Syncom IV (Leasat)
Hughes Communications Service is
building and operating communications
satellites for lease to the US Department
of Defence. The Leasat (US Navy desig
nation Syncom IV) network will comprise
four satellites in geostationary orbit, plus
a spare.
Purpose Communications satellite
Payload 12 UHF transponders
1 SHF transponder
Owner Hughes Communications Ser
vices (leased to the US Navy)
Orbit Geostationary
Launch
Leasat/Syncom IV-2 Aug 1984 Shuttle
1 Nov 1984 Shuttle
3 Mar 1985 Shuttle
4 Aug 1985 Shuttle
Stabilisation Spin
Power (end of life) l-2kW
Weight (at launch) 6,580kg
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